Poly(neopentylglycol)azelate (NPGA) is a polymer used in certain polyurethane binders for composite solid propellants.
Historically, in processing polyurethane propellants, all the binder ingredients, with the exception of the curing agent are combined in a submix. The solids including the cure catalyst are added to the submix after which the curing agent is added which cures the polyurethane propellant to a solid. NPGA as a propellant ingredient has contributed to desirable propellant ballistic properties but also occasionally has caused processing difficulties and poor mechanical properties. It has been determined that the unfavorable processing and the poor mechanical properties are related to the fact that NPGA has limited solubility in the mixture of other binder ingredients. As a result of lower NPGA solubility in the binder, processing problems have arisen as a result of increased propellant mix viscosity. The principal mechanical property effects are increased modulus at low temperatures, which imposes greater stresses on rocket motor bondlines, and decreased tensile strength, which reduces the propellant's capability to survive the greater stresses. The inferior propellant can then separate from rocket chamber liners or insulators at bondlines, thus causing anomalous motor performance and mission failure.
The correlation between the solubility of the NPGA and a normal variation in its production process has been established. The variations in processing causes a variation of NPGA molecular weight and/or hydroxyl content. The solubility of the NPGA in the submix decreases as its molecular weight increases and/or hydroxyl content decreases. Since the molecular weight and/or hydroxyl content are a result of normal variations in the production process, a different approach to solving the problem of decreased NPGA solubility in propellant binders is required other than controlling the production processing parameters for NPGA.
An object of this invention is to provide a method to ensure adequate solubility of NPGA in a polyurethane propellant binder.
Another object of this invention is to provide a method to ensure adequate solubility of NPGA in a polyurethane propellant binder as evidenced by effects on propellant viscosity and mechanical properties even when the NPGA contains a large high molecular weight fraction.